Big Tunnels is one of the better dives on the west side of Grand Cayman. It has a large passageway, “tunnel” that is wide enough for a couple of buses to pass side by side and dumps out to the wall at about 100 feet. There are a number of spiral routes up through the coral where you can find a wide variety of marine life including a number of Tarpon, usually 
congregated in one of the passages ways just down from the top of the wall.
You will find Orange Tube sponges, Spiny Lobsters, several small Bubble-tip Anemone colonies, Schoolmasters, Green Sea turtles as well as Hawksbill in the area and much more. You may also see a Spotted Eagle Ray off the wall, so make sure you take the chance to “peer into the blue” as you are swimming along the wall. The site can have current as it is close to the west end of the island and it can also be fairly turbid at times. However, most of the time you will find good to excellent visibility and a good spot for underwater photography.
As you exit the tunnel on the wall you can usually get a great shot of the wall with the sun back lighting the top of the wall. When taking wide angel shots at Big Tunnels, I would recommend having your strobes are aimed “out” as you can pick up a good bit of back scatter.
As you come up to the top of the wall you will find a good variety of health soft and hard corals, including rope sponges, brain corals, star corals, barrel sponges, sea whips as well as a variety of 
crustaceans. This is a very relaxed dive with good variety typical of the west wall. Overall I rate this site highly, but I tend to prefer Bonnie’s Arch and Northwest Point (if the current is not bad) and Orange Canyon.